1. Reaction Mechanisms & Rate Determining Step

2. Learning Intentions What do we mean by a reaction mechanism?
What is a multi step reaction?
In a multi step reaction what determines the rate of a reaction?
How can we work out the RDS?

3. Reactants  Products
This tells us nothing about how we form the products

4. Reaction Mechanisms Consider the following: 2O3 (g)  3O2 (g)
This tells us the reactants and products, but
gives no information on how we form the
products from the reactants

5. Possible Mechanisms: 2O3 (g)  3O2 (g)
2 ozone molecules collide in the correct orientation to produce 3 oxygen molecules
Proposed Mechanism = 2O3 (g)  3O2 (g) O O O O O O O O O O O O

6. Possible Mechanisms 2O3 (g)  3O2 (g)
2) One ozone molecule “kicks out” an oxygen atom O O O O O O
This “spare” oxygen atom came across a different ozone molecule and reacts with it to form two O2 molecules O O O O O O O O

7. Possible Mechanisms: 2O3 (g)  3O2 (g)
Proposed Mechanism
This one has two stages – it is a multi-step mechanism
O3  O2 + O
O3 + O  2O2
Overall = both added together
O3 + O3 + O  O2 + O2 + O2 + O
= 2O3  3O2
As this is the same as our overall reaction equation it
can be considered a valid proposed mechanism

8. Learning Intentions What do we mean by reaction mechanism?
What is a multi step reaction?
In a multi step reaction what determines the rate of a reaction?
How can we work out the RDS?

9. What determines the rate of a reaction?
Reading:
3 mins
Wrapping:
10 mins
Delivering:
5 mins
In 30 mins how many
letters have been read?
presents wrapped?
presents delivered?
What determines the
rate of reaction?

10. Rate Determining Step
The rate of a reaction is determined by the slowest stage of the mechanism.
This is called the rate determining step

11. Learning Intentions What do we mean by reaction mechanism?
What is a multi step reaction?
In a multi step reaction what determines the rate of a reaction?
How can we work out the RDS?

12. How can we work out the rate determining step?
Consider the reaction:
(CH3)3CBr + OH– → (CH3)3COH + Br–
Mechanism:
(i) (CH3)3CBr → (CH3)3C+ + Br–
(ii) (CH3)3C+ + OH– → (CH3)3COH
SLOW
FAST
rate = k[(CH3)3CBr] (will be given if asked to find RDS)
The order of OH- is 0 (doesn’t affect the rate)
Therefore OH- not involved in the rate determining step

13. How can we work out the rate equation from mechanism?
Consider the reaction:
H2O2 + HI → H2O + HOI SLOW
HOI + HI → H2O + I2 FAST
Here RDS contains H2O2 + HI
(RDS will be given)
rate = k[H2O2][HI]

14. The order of reaction with respect to a reactant and the number of molecules in the rate determining step
Overall Reaction: 2X + 3Y  Z Step 1: 2X + Y  Q (SLOW) Step 2: Q + 2Y  Z (FAST)

15. Learning Intentions What do we mean by reaction mechanism?
What is a multi step reaction?
In a multi step reaction what determines the rate of a reaction?
How can we work out the RDS/rate equation?